There are a lot of options. You could, for example, listen to Alicia Keyes' free YouTube concert. You could go to Starbucks and buy a Product(RED) gift card. You could volunteer with an AIDS Service Organization. You could get tested. You could talk to someone about AIDS.
All of which are fine things to do. But in my view they ignore the two fundamental causes of AIDS: the HIV virus, and injustice. The way AIDS works is that you get infected by the HIV virus, and it makes you sick. Getting infected can happen to anyone, but it happens disproportionately to people who are stigmatized, poor, and disenfranchised. The solutions to preventing AIDS are political solutions. Stopping politicians from keeping clean needles out of the hands of injection drug users. Stopping politicians from keeping condoms away from prisoners and young people. Stopping politicians from criminalizing and stigmatizing homosexuality. Forcing politicians to face and solve the problem of homelessness. Forcing politicians to put people's lives ahead of drug company profits. Forcing politicians to make sure everyone has access to affordable health care. Access to treatment, housing, health care, clean needles, condoms, justice, and enfranchisement are scientifically proven to fight transmission of the virus that causes AIDS. And politicians and corporations are the main things standing in our way. The spread of AIDS is not the fault of drug users, LGBTQ people, people who have sex, or mothers. The spread of AIDS is the fault of people unwilling to spend the money or political capital to solve the problem.
The other cause of AIDS is the HIV virus. Stopping viruses requires treatment and research. Which cost money. Which brings us back to the above point.
So this World AIDS Day, in addition to buying your Product (RED) coffee, listening to Alicia Keyes, or volunteering, take time to work for the political solution.
Contact Mayor Michael Nutter and tell him a 2-year AIDS housing waiting list is unacceptable: http://org2.democracyinaction.org/o/5712/p/dia/action/public/?action_KEY=1343
Contact State AIDS Director Joseph Pease and let him know that returning or not applying for nearly $15 million dollars of state funding is unacceptable: http://org2.democracyinaction.org/o/5712/t/8185/p/dia/action/public/?action_KEY=1255
Contact President Obama and let him know that earning a D+ on fighting AIDS is unacceptable, and that you expect him to achieve full marks next term (don't worry, he still can. He just needs your help to make this a hot political issue!) http://healthgap.org/press/wad09_report_card_pr.htm